Container refrigeration apparatuses including a refrigerant circuit performing a refrigerant cycle have been used to cool air in a container for use in, e.g., marine transportation (see, e.g., Patent Document 1). The container is loaded with plants such as bananas and avocados. Plants perform respiration by absorbing oxygen in the air and releasing carbon dioxide even after they are harvested. The plants lose their nourishment and moisture stored in them as they respire. Thus, if the respiration rate of the plants increases, the degree of freshness of the plants decreases significantly. On the other hand, if the oxygen concentration of the air in the container decreases to a predetermined concentration as a result of the plant respiration, the respiration rate of the plants decreases. However, since it takes more time to reach such a predetermined concentration only through the respiration of the plants, the plants will be discolored, rotten in the meantime to have a decreased degree of freshness.
Patent Document 1 discloses a refrigeration apparatus in which the oxygen concentration of the air in a container is quickly reduced by separating nitrogen from the air with a membrane separator to produce nitrogen-enriched air, and supplying such nitrogen-enriched air into the container. As can be seen, if the oxygen concentration of the air in the container is set to be lower than that of the outside air, the respiration rate of the plants may be reduced so much that the degree of freshness can be maintained more easily.